Various methods are currently, utilized for the removal of small particles from surfaces, such as a surface of a wafer in the fabrication of integrated circuit devices. Particles or contaminates are removed by a variety of mechanisms including ultrasonic techniques, high pressure spraying techniques, mechanical scrubbing techniques etc. Such particle or contaminant removal is commonly carried out after a variety of process steps and before carrying out other process steps. For example, particle or contaminant removal, i.e., cleaning, is necessary after the performance of sawing, lapping, planarization, polishing, and before other device fabrication steps are performed, such as metalization, chemical vapor deposition, epitaxy, etc. Further, and more specifically, for example, after planarization has been performed, such as with the use of a slurry including silica or alumina particles, a post planarization clean is performed prior to carrying out other fabrication steps. The small particles or contaminants resulting from the fabrication steps above, tenaciously hold to a surface and typically require relatively large forces to remove them, in part due to the electrostatic potential of such particles.
The use of ultrasonic energy to enhance the cleaning action of solutions used to clean semiconductor wafer surfaces is well known. Such ultrasonic agitation provided by the ultrasonic energy typically is provided by using transducers operating at intermediate frequencies. i.e., 20-50 KHz. With the use of ultrasonic energy, bubbles are formed in the cleaning solution in which the wafers are immersed and the bubbles collapse under the pressure of the ultrasonic agitation. This produces shock waves which impinge on the wafer surfaces. The bubble collapsing is known as cavitation. The shock waves dislodge and displace particles to be carried away in the cleaning solution.
Further, the use of high frequency, or megasonic frequencies, for example, in the range of 0.2-5.0 MHz, is also well-known. The use of such high frequencies has also resulted in improved cleaning, particularly on substrates or wafers with very small, micron size elements disposed thereon. Further, use of such high frequencies provides a gentler cleaning action on the wafers and/or substrates than is attainable with an intermediate frequency. Therefore, such gentler cleaning action results in less damage during cleaning operations.
Use of megasonic energy is more efficient than ultrasonic cleaning for submicrometer particle removal because it functions via a different mechanism than ultrasonic cavitation. Because megasonic energy occurs at higher frequencies than ultrasonic energy, the pressure wave that forms generates a pulse so rapidly that the vacuum bubbles do not have time to form as in the use of ultrasonic energy. Consequently, megasonic energy consists of a series of pressure waves. When applied parallel to a surface, these waves dislodge particles. Usually this occurs by allowing a thin film of the cleaning solution, such as deionized water, to form between the particle and the surface being cleaned, thereby reducing the attraction between the surface and the particle to facilitate removal of the particle.
One illustration of a current method utilized for cleaning wafer surfaces, such as after a planarization process is performed, includes the immersion of the wafers in deionized water while megasonic energy is projected therethrough. During the cleaning of the wafers, the tank in which the liquid is held, is dumped and refilled one or more times. This dumping and refilling appears to draw in air and create an air/liquid interface across the surface of the wafer which appears to enhance the megasonic cleaning method. However, dumping and refilling the megasonic bath consumes a large amount of the deionized water and/or whatever chemicals might be alternatively utilized as the liquid for performing the cleaning of the wafer. Further, performing such dumping and refilling operations is also an inefficient use of time.
Also, in performing cleaning operations, it is in some circumstances, desirable to control the pH of the liquid held by the cleaning tank in which the wafers are immersed. For example, after a highly acidic or basic clean has been performed, a deionized water clean is then typically used before proceeding with any further process steps. However, it may not be desirable to perform a neutral deionized water clean immediately following a highly acidic or basic clean. Rather, it may be more desirable to use an acidic or basic deionized water bath.
Accordingly, there is need in the art for alternative megasonic cleaning methods which overcome the disadvantages as described above. The present invention overcomes these problems and overcomes other problems as will become apparent to one skilled in the art from the description below.